Micro-electromechanical systems (MEMS) are used in a variety of applications, including optical display systems. Such systems often include a light source that projects light rays onto a light modulator panel. In many optical display systems, the light modulator panel includes an array of MEMS devices commonly referred to as pixels. The pixels modulate light to control its color, intensity, hue, or other characteristics.
For example, some pixels include reflective plates that are selectively tilted to direct light along a desired path. More specifically, when such pixels are in an ON state, the reflective plate is tilted such that the pixels direct light incident thereon to the display optics, which focus the light onto a display surface. Similarly, when such pixels are in an OFF state, the reflective plate is oriented such that the light is directed away from the display surface. By controlling the frequency with which the light is directed from each pixel to the display surface, each pixel is able to produce an output that varies from light to dark on the display surface. By properly controlling an array of pixels, a full image may be formed.
Some systems make use of a prism to separate the display light from the non-display light. In particular, such prisms frequently include a single, total internal reflection interface to direct light from the light source to the light modulator panel. The light modulator panel then directs the display light back through the prism and to the display surface while the tilt of the mirrors directs the non-display light away from the display surface. The degree to which the display light and non-display light are separated depends in large part on the change in angle of the reflective plates between an ON state and an OFF state.
Further, according to such systems the back focal distance, or the distance from the modulator to the display optics, may depend on where light rays from the edges of the light modulator panel intersect. In particular, a back focal distance in which the display light and non-display light are sufficiently separated may depend on the point where a non-display ray from the edge of the light modulator panel nearest the light source intersects a display edge from the opposite edge of the light modulator panel. The intersection of these rays may represent the point at which the non-display and display light are separated. Thus, placing the back focal distance at a location beyond the point of intersection may help ensure that non-display light is not directed to the display optics. The location of this point of intersection frequently depends on the angle of rotation of the reflective plates at their ON and OFF states. As a result, systems making use of reflective plates with relatively small angles of rotation and a single reflection interface may have relatively large back focal distances. Relatively large focal distances may correspond to relatively large systems.